Light-emitting devices using LED chips (LED devices) are finding wide applications because of increasing demand for light-emitting devices with higher luminance and lower energy consumption. Among other LED devices, white LED devices are particularly known that use a combination of a blue LED chip and a yellow phosphor that emits yellow light upon receipt of blue light allowing the blue light and the yellow light to be mixed so that white light is emitted from the device. The white LED devices are increasingly used as lighting sources such as electric lights, which require white light, and backlights for liquid crystal displays.
Other types of white LED devices that use a combination of an LED chip and phosphor have also been studied, including white LED devices that create white light by the combination of a LED chip that emits UV light and phosphors that emit, upon receipt of UV light, blue, green and red lights, and white LED devices that create white light by the combination of a LED chip that emits blue light and phosphors that emit red and green lights.
The white LED devices that use a combination of LED chip and phosphor can generate white light with a single LED chip and therefore can be more simplified as compared to white LED devices that use a combination of a plurality of LED chips of different colors for generation of white light. Moreover, for their low power consumption, the white LED devices have been advantageously used.
However, the light from white LED devices using a combination of LED chip and phosphor is undesirably colored when a balance between the emission light from the LED chip and fluorescence from the phosphor is disrupted. Coloring of light from the white LED device also causes “color non-uniformity,” a phenomenon where color (chromaticity) varies depending on the observation angle of the device.
One cause of light coloring and color non-uniformity pertinent in the white LED device is the non-uniform distribution of phosphors in the LED device. The conventional manufacturing process of a white LED device involves applying on the LED chip a curable resin composition containing dispersed phosphor particles followed by curing, to form a phosphor layer around the LED chip. Phosphor, however, is generally an inorganic metal compound having an extremely high specific gravity; therefore, the phosphors in the curable resin composition settle down resulting in the phosphor particles to be more likely deposited on the LED chip in a non-uniform manner. As a consequence, light coloring and/or color non-uniformity occur in the white LED device.
Techniques such as those described below have been contemplated in an effort to reduce the occurrence of light coloring and/or color non-uniformity.
One disclosed technique for reduced color non-uniformity in the white LED device involves adding an anti-settling agent to a liquid encapsulating material containing phosphor particles thus preventing settling of the phosphor particles, which have high specific gravity (see PTL 1). Moreover, formation of a phosphor-containing encapsulating layer around the LED chip is followed by curing of the encapsulation layer while spinning the light-emitting device thus reducing the chromaticity difference, i.e., color non-uniformity in the light-emitting device (see PTL 1).
Another disclosed technique involves deposition of phosphor particles on the emission surface of the LED chip upon manufacture of a white LED device (see PTL 2). PTL 2 particularly claims that reduced conversion efficiency as well as chromaticity deviation among different directions and color non-uniformity due to failure to apply phosphor particles on the corners and lateral surfaces of the LED chip can be improved by spraying a spiral of atomized phosphor-containing coating solution.